One day, the U.S. military hopes to use tiny flying robots, equipped with cameras and sensors, for surveillance. But such robots would need to be able to navigate around obstacles, carry weight, and be efficient enough to fly for long periods of time. A group of researchers believe that the key to making such a robot might lie in the dragonfly.
Dragonflies are one of few creatures that utilize four independently controlled wings to fly, allowing them to hover, dart, glide, move backward, and change directions rapidly. Looking to understand such abilities, scientists at the Royal Veterinary College, in England, and the University of Ulm, in Germany, have developed a robotic dragonfly to measure the current flows over and under the wings at different flap cycles. While most of the dragonfly hovering scenarios were not efficient, the team found that if the lower wings are beating slightly ahead of the top wings, the double set of wings proves more efficient at generating lift, employing 22 percent less power to lift the same weight as a single pair.
“The one specific advantage you get in four wings is the maneuverability and ability to pick things out of the air and hover and dart around,” says Jonathan How, a professor at MIT who works on flying robots but was not involved in the dragonfly project. “It would be really amazing if we could build something that got anywhere near that level of performance. If you can achieve the same lift at a lower power, that’s helpful.”
Despite their potential advantages, small flying robots that mimic dragonflies’ agility have not been successfully made, in part because of the complexity of the aerodynamics around four wings, and also because of fabrication issues involved with small flying machines. However, studies of wing motion and air forces that reveal how dragonflies achieve their agility may enable roboticists to eventually build capable, swift flyers that use four wings.
Watch a robotic dragonfly’s wing motion when submerged in oil.
To measure the air currents, the Ulm researchers immersed the robotic dragonfly in a tank filled with mineral oil and peppered with air bubbles. Two green lasers combined and reflected off the air bubbles as a high-speed camera took images 10 to 20 milliseconds apart. By comparing images, the scientists calculated the direction of flow for regions within the tank.
In terms of four-wing versus two-wing systems for a biomimetic micro air vehicle, “it’s a trade-off,” says Fritz-Olaf Lehmann, a researcher at the University of Ulm who worked on the study. With a four-wing system, the disadvantages are the need for an extra control system and extra power. However, a system with two wings must incorporate ways to change the angle, amplitude, and frequency of the wings flapping to change lift, says Lehmann. Conversely, with four wings, “you can just advance one flight system against the other, and then you change lift production,” he says. “I think that makes building a micro air vehicle much easier.”
In creating an autonomous micro air vehicle, “every little bit of efficiency counts,” says Robert Wood, a professor at Harvard University who has developed some of the smallest flying robots. “You could make the argument that if you have a four-winged vehicle, you’ll have more [control] to assist you in stabilization,” he says.
Michael Dickinson, a professor at Caltech who works on understanding and mimicking fly flight, says that interest in dragonflies is growing and that the Lehmann paper is not the first containing this kind of analysis but “one in a floodgate of papers.” While the study might add to the understanding of the subtle aerodynamics of four-winged flight, Dickinson points out that researchers must still develop a better, lighter battery that powers the vehicle and makes an effective control system.